1. Field of the Invention
This invention relates to an ink jet recording apparatus, and in particular to an ink jet recording apparatus provided with a fixating heater for expediting the fixation of ink adhering as recorded images to a recording medium.
2. Related Background Art
An ink jet recording apparatus-has many merits such as very small noise during recording and the possibility of high-density recording, and has been drawing growing attention as a recording apparatus in recent years.
Also, the ink jet recording apparatus has a great merit in that plain paper can be chosen as a recording medium, but depending on the combination of recording paper and ink used, there may sometimes arise a problem as to the fixativeness of ink on the recording paper. That is, the ink jet recording apparatus uses as a recording agent ink which is liquid and therefore, where the ink absorbing property of the recording medium for the ink used is inferior, the ink does not quickly permeate into the recording medium and the ink which cannot be absorbed into the recording medium remains in the form of liquid on the recording medium.
By such a condition occurring, conveying members such as rollers are contaminated and such ink is transferred to the surface of a succeeding recording medium, which has sometimes led to remarkable deterioration of the recording quality. Further, when the operator handles a recorded recording medium on which the fixation of ink is insufficient, the ink not only has contaminated the operator's hands, but the hands have also inadvertently rubbed the formed image thereby disturbing the quality of the image. Or in the case of an apparatus in which recorded recording mediums are piled on a discharge tray or the like, there have sometimes been caused problems, such as deterioration of the quality of the image and contamination by the rubbing between the recording mediums.
As a technique for preventing such problems and enhancing the fixativeness of ink on the recording medium, there is an apparatus in which a heater is used to heat the recording medium and expedite the evaporation of the water content contained in ink, thereby causing the ink on the recording medium to be quickly dried and fixated. From the requirements for shortening the length of the conveyance path of the recording medium and quickening the fixation of the ink for the prevention of the contamination of conveying members, the fixating heater is disposed near a recording station by a recording head, for example, on a platen opposed to the recording head for controlling the recording surface of the recording medium so as to be flat. Also, with attention being paid to the fact that if the recording medium is dry, the ink absorbing property thereof is enhanced, a heater is sometimes provided upstream of the recording station in the conveyance path to preheat and dry the recording medium.
A further cause which disturbs the quality of recorded images, is the problem of moisture condensation, such as dew formation in the ink jet recording apparatus. That is, the problem is that in the environment in which the apparatus is used, water content or the like which has condensed in the apparatus adheres to the recording medium to cause the ink to blot on the recording medium. The problem resulting from such moisture condensation could not be solved simply by providing a fixating heater.
So, the inventors have focused their attention on the electrophotographic technique which is a different field of art, in order to prevent moisture condensation, and have conceived of the application of a moisture condensation preventing device used in an electrophotographic copying apparatus to an ink jet recording apparatus. It is, for example, the method described in Japanese Laid-open Pat. Application No. 55-35390 wherein a plurality of temperature sensors, a humidity sensor and a plurality of heaters provided in a paper supply portion, an optical system and a fixating device, respectively, are heat-controlled, or the method described in Japanese Laid-open Pat. Application No. 56-80061 wherein in addition to said method, removal of humidity is effected by a blower.
However, simple application of these methods to an ink jet recording apparatus has posed several problems: for example, (1) the provision of a plurality of moisture condensation preventing heaters in addition to the fixating heater makes the entire apparatus bulky; and (2) the heat from the moisture condensation preventing heaters affects the viscosity of ink to reduce the stability of the recording characteristic. In addition, it has been made clear on the basis of the inventors' experiment that the causes of the moisture condensation occurring in the ink jet recording apparatus and the influence thereof are due chiefly to the reasons set forth below.
That is, according to the inventors' experiment, it is at a temperature on the order of 80.degree.-60.degree. C. of the recording medium that no warp occurs to the recording medium and, moreover, good and quick fixation takes place. On the other hand, the recording head is kept on the order of 20.degree.-40.degree. C. with parameters which affect the stability of discharge, such as the viscosity of ink being taken into account, and this temperature is lower than the ambient temperature of the recording head.
Since the apparatus is operated under such environmental conditions, the recording head (particularly in the vicinity of its discharge ports) becomes a high-humidity atmosphere due to the evaporation of water content from the recording medium and the evaporation of the water content contained in the ink caused by the heating for fixation, and moisture condensation occurs on the surface of the recording head in which the ink discharge ports are provided (hereinafter referred to as the discharge surface). Generally, in the ink jet recording system, in order to improve the quality of recording, it is required that ink droplets adhere to a desired position on the recording surface, that is, the direction of discharge and flight of ink droplets used for recording should be constant, the speed of discharge should be constant and the particle diameters of the flying ink droplets should be uniform.
However, if moisture condensation occurs on the discharge surface, particularly, around the discharge ports, water drops or the like adhere non-uniformly to the portion around the discharge ports, and when the ink is discharged from the ink discharge ports, such water drops pull the discharged ink non-uniformly and therefore, the direction of discharge, the speed of discharge and the particle diameters of the ink droplets become irregular, thus resulting in the deterioration of the quality of recording. Also, the wetting of the discharge surface makes paper powder, dust or the like readier to adhere thereto, and this causes the deterioration of the quality of recording.
As described above, it has been found, as a result of numerous experiments carried out by the inventors and the detailed studies thereof, that the problem of moisture condensation cannot be solved unless consideration, is fully given to optimum conditions such as the fixation temperature in the ink jet recording apparatus and the temperature of the recording head.